It has been found that computed tomagraphy (CT) imaging and computer modelling methods provide a precision for pre-surgical planning, simulation, and custom implant design that greatly exceeds the precision of subsequent surgical execution. For example, approximately one half of the 300,000 total hip replacement operations performed each year use cementless implants. Stability of the implant, uniform stress transfer from the implant to the bone, and restoration of the proper biomechanics critically affect efficacy and, in turn, are significantly affected by the proper placement of the implant relative to the bone. An important factor in achieving proper placement of the implant is the accuracy with which the femoral cavity is prepared to match the implant shape.
Recently reported research confirms that gaps between implant and bone significantly affect bone ingrowth. One study of the standard manual broaching method for preparing the femoral cavity found that the gaps between the implant and the bone is commonly in the range of one millimeter to four millimeters and that the overall resulting hole size was 36% larger than the broach used to form the hole. As a result, only 18-20 percent of the implant actually touches bone when it is inserted into such a hole. Furthermore, the placement of the implant cavity in the bone, which affects restoration of biomechanics, is as much a function where the broach "seats" itself as of any active placement decision on the part of the surgeon.
Typically, precise surgical execution has been limited to procedures, such as brain biopsies, for which a suitable stereotactic frame is available. However, the inconvenience and restricted applicability of these devices has led some researchers to explore the use of robots to augment a surgeon's ability to perform geometrically precise tasks planned from CT or other image data.
Safety is an obvious consideration whenever a moving device such as a robot is used in the vicinity of a patient. In some applications, the robot does not need to move during the "in-contact" part of the procedure. In these applications the robot moves a passive tool guide or holder to a desired position and orientation relative to the patient. Brakes are then set and motor power is turned off while a surgeon provides whatever motive force is needed for the surgical instruments. Other surgical applications rely on instrumented passive devices to provide feedback to the surgeon on where the instrument is located relative to an image-based surgical plan.
In an IBM Research Report (RC 14504 (#64956) 3/28/89) R. H. Taylor et al. describe a robotic system for milling a correctly shaped hole into a femur for receiving a cementless hip implant. The system computes a transformation between CT-based bone coordinate data and robot cutter coordinates. The transformation is accomplished in part by a combination of guiding and tactile search used to locate a top center of each of three alignment pins that are pre-surgically affixed to the femur and CT-imaged. This robotic system includes a vision subsystem to provide a redundant check of the robot's motion to ensure that the tool path does not stray outside of a planned work volume. An online display is provided for the surgeon to monitor the progress of the operation. Proximity sensors may be positioned to detect any subsequent motion of the pins relative to a robot base.